arXiv subjects

A. V. Sergeev

Publications and source records attributed to A. V. Sergeev.

13 recordsLinked to original sources

arXiv2020

Two-component electrolyte solutions with dipolar cations on a charged electrode: Theory and computer simulations

The development of advanced electrochemical devices for energy conversion and storage requires fine tuning of electrode reactions, which can be accomplished by altering the electrode/solution interface structure. Particularly, in case of an alkali-salt electrolyte the electric double layer (EDL) composition can be managed by introducing organic cations (e.g. room temperature ionic liquid cations) that may possess polar fragments. To explore this approach, we develop a theoretical model predicting the efficient replacement of simple (alkali) cations with dipolar (organic) ones within the EDL. For the typical values of the molecular dipole moment ($2-4~D$) the effect manifests itself at the surface charge densities higher than 30 $μC/cm^2$. We show that the predicted behavior of the system is in qualitative agreement with the molecular dynamics simulation results.

cond-mat.soft↗

arXiv2018

GD358: three decades of observations for the in-depth asteroseismology of a DBV star

We report on the analysis of 34 years of photometric observations of the pulsating helium atmosphere white dwarf GD358. The complete data set includes archival data from 1982-2006, and 1195.2 hours of new observations from 2007- 2016. From this data set, we extract 15 frequencies representing g-mode pulsation modes, adding 4 modes to the 11 modes known previously. We present evidence that these 15 modes are ell = 1 modes, 13 of which belong to a consecutive sequence in radial overtone k. We perform a detailed asteroseismic analysis using models that include parameterized, complex carbon and oxygen core composition profiles to fit the periods. Recent spectroscopic analyses place GD358 near the red edge of the DBV instability strip, at 24,000 plus or minus 500 K and a log g of 7.8 plus or minus 0.08 dex. The surface gravity translates to a mass range of 0.455 to 0.540 solar masses. Our best fit model has a temperature of 23,650 K and a mass of 0.5706 solar masses. That is slightly more massive than suggested by most the recent spectroscopy. We find a pure helium layer mass of 10^-5.50, consistent with the result of previous studies and the outward diffusion of helium over time.

astro-ph.SR↗

arXiv2015

Study of the characteristics of SiPMs matrix as a photosensor for the scintillation detectors

The matrices formed of silicon photomultipliers (SiPMs) are very promising photosensors for the scintillation detectors. The use of SiPM matrices with appropriate optical collector gives, in principle, a possibility to do a snapshot of glowing track of charged particle traversing a scintillator. The prototype of such scintillation detector is under development now in INR RAS. The preliminary results of characterization study of the matrix ArrayC-60035-64P-PCB (SensL company) for the prototype of such detector are presented.

astro-ph.IM↗

arXiv2012

Empirical Determination of Convection Parameters in White Dwarfs I : Whole Earth Telescope Observations of EC14012-1446

We report on analysis of 308.3 hrs of high speed photometry targeting the pulsating DA white dwarf EC14012-1446. The data were acquired with the Whole Earth Telescope (WET) during the 2008 international observing run XCOV26. The Fourier transform of the light curve contains 19 independent frequencies and numerous combination frequencies. The dominant peaks are 1633.907, 1887.404, and 2504.897 microHz. Our analysis of the combination amplitudes reveals that the parent frequencies are consistent with modes of spherical degree l=1. The combination amplitudes also provide m identifications for the largest amplitude parent frequencies. Our seismology analysis, which includes 2004--2007 archival data, confirms these identifications, provides constraints on additional frequencies, and finds an average period spacing of 41 s. Building on this foundation, we present nonlinear fits to high signal-to-noise light curves from the SOAR 4.1m, McDonald 2.1m, and KPNO 2m telescopes. The fits indicate a time-averaged convective response timescale of 99.4 +/- 17 s, a temperature exponent 85 +/- 6.2 and an inclination angle of 32.9 +/- 3.2 degrees. We present our current empirical map of the convective response timescale across the DA instability strip.

astro-ph.SR↗

arXiv2011

Detection of high-frequency variability in chromospherically active stars

We have carried out high-speed photometry of three chromospherically active stars, BD +15 3364, II Peg, and SAO 52355 with the Zeiss 2-m telescope, as well as low-resolution spectroscopy of SAO 52355 with the Zeiss-600 telescopes at Peak Terskol. BD +15 3364 is known as chromospherically active star of the spectral type G0. II Peg is the RS CVn binary. It has a spectral type K2IV-Ve. SAO 52355 is a field star of the spectral type K0 III. The X-ray observations taken by Ginga and ROSAT indicate in II Peg and SAO 52355 coronae plasma temperatures as high as $10^{7}$ K. These two stars are supposed to have high-powered chromospheres. Photometric observations of all three stars show high-frequency variations in brightness in the UBV bands at subsecond range. Intensity variations are found peaked at frequency around about 0.5 Hz, spanning the range up to 1.5 Hz for BD +15 3364, II Peg, and up to about 35 Hz for SAO 52355. The relative power of fluctuations reaches ($10^{-3.7} - 10^{-4.2}$) in the UBV bands. Spectroscopic monitoring of SAO 52355 showed variations of emission in the Balmer lines and in the CaII H, K lines at time intervals ranging from seconds to minutes. From the power spectrum data one can find that variations in the intensities of the CaII H, K and $H_γ$ lines are 3.2% and 1.5%, respectively. This allows us to assert the existence of intense microflaring activity in these stars.

astro-ph.SR↗

arXiv2010

Formation of asteroid pairs by rotational fission

Asteroid pairs sharing similar heliocentric orbits were found recently. Backward integrations of their orbits indicated that they separated gently with low relative velocities, but did not provide additional insight into their formation mechanism. A previously hypothesized rotational fission process4 may explain their formation - critical predictions are that the mass ratios are less than about 0.2 and, as the mass ratio approaches this upper limit, the spin period of the larger body becomes long. Here we report photometric observations of a sample of asteroid pairs revealing that primaries of pairs with mass ratios much less than 0.2 rotate rapidly, near their critical fission frequency. As the mass ratio approaches 0.2, the primary period grows long. This occurs as the total energy of the system approaches zero requiring the asteroid pair to extract an increasing fraction of energy from the primary's spin in order to escape. We do not find asteroid pairs with mass ratios larger than 0.2. Rotationally fissioned systems beyond this limit have insufficient energy to disrupt. We conclude that asteroid pairs are formed by the rotational fission of a parent asteroid into a proto-binary system which subsequently disrupts under its own internal system dynamics soon after formation.

astro-ph.EP↗

arXiv2009

Experimental search of bursts of very high energy gamma rays from primordial black holes

The technical procedure of a search of bursts of very high energy gamma rays from evaporation of primordial black holes on air-shower array "Andyrchy" of Baksan Neutrino Observatory of Institute for Nuclear Research is described. The theoretical model used in the present work assumes that the chromosphere around the evaporating black hole does not form. For minimization of the cosmic ray background the method of multidimensional analysis of modelled as well as experimentally detected events is applied. The new upper limit on the concentration of evaporating primordial black holes in the local region of Galaxy is obtained. The comparison of the results of different experiments is given.

astro-ph.CO↗

arXiv2009

Experimental search of bursts of gamma rays from primordial black holes using different evaporation models

Experimental data of arrays "Andyrchy" and "Carpet-2" of Baksan Neutrino Observatory (Institute for Nuclear Research), obtained in the regime of a detection of the single cosmic-ray component, are used for a search of the bursts of cosmic gamma rays from evaporating primordial black holes. Different theoretical models of the evaporation process are used for the analysis. Distributions of the counting rate fluctuations on both arrays agree with the expectations from the cosmic ray background. The new constraints on the concentration of evaporating primordial black holes in the local region of Galaxy are obtained. The comparison of the results of different experiments is given.

astro-ph.CO↗

arXiv2008

2006 Whole Earth Telescope Observations of GD358: A New Look at the Prototype DBV

We report on the analysis of 436.1 hrs of nearly continuous high-speed photometry on the pulsating DB white dwarf GD358 acquired with the Whole Earth Telescope (WET) during the 2006 international observing run, designated XCOV25. The Fourier transform (FT) of the light curve contains power between 1000 to 4000 microHz, with the dominant peak at 1234 microHz. We find 27 independent frequencies distributed in 10 modes, as well as numerous combination frequencies. Our discussion focuses on a new asteroseismological analysis of GD358, incorporating the 2006 data set and drawing on 24 years of archival observations. Our results reveal that, while the general frequency locations of the identified modes are consistent throughout the years, the multiplet structure is complex and cannot be interpreted simply as l=1 modes in the limit of slow rotation. The high k multiplets exhibit significant variability in structure, amplitude and frequency. Any identification of the m components for the high k multiplets is highly suspect. The k=9 and 8 modes typically do show triplet structure more consistent with theoretical expectations. The frequencies and amplitudes exhibit some variability, but much less than the high k modes. Analysis of the k=9 and 8 multiplet splittings from 1990 to 2008 reveal a long-term change in multiplet splittings coinciding with the 1996 "sforzando" event, where GD358 dramatically altered its pulsation characteristics on a timescale of hours. We explore potential implications, including the possible connections between convection and/or magnetic fields and pulsations. We suggest future investigations, including theoretical investigations of the relationship between magnetic fields, pulsation, growth rates, and convection.

astro-ph↗

arXiv2008

Searching for Very-High-Energy Gamma-Ray Bursts from Evaporating Primordial Black Holes

Temporal and energy characteristics of the very-high-energy gamma-ray bursts from evaporating primordial black holes have been calculated by assuming that the photospheric and chromospheric effects are negligible. The technique of searching for such bursts on shower arrays is described. We show that the burst time profile and the array dead time should be taken into account to interpret experimental data. Based on data from the Andyrchy array of the Baksan Neutrino Observatory (Institute for Nuclear Research, Russian Academy of Sciences), we have obtained an upper limit on the number density of evaporating primordial black holes in a local region of space with a scale size of ~10^{-3} pc. Comparison with the results of previous experiments is made.

astro-ph↗

arXiv2007

Ukrainian Synchronous Network of small Internet Telescopes as rapid action instrument for transient objects

UNIT (The Ukrainian synchronous Network of small Internet Telescopes) is a system of automated telescopes that search for simultaneous optical activity of transient objects associated with variable stars, small bodies of the Solar system, Near-Earth objects (NEOs), gamma ray bursts, etc. Their instruments are sensitive down to $M_{V} \approx 18$ and require an average of 60 seconds to obtain the first images of the transient events after the alarm or GCN notice. Telescopes of UNIT are equipped with fast CCD cameras to study astrophysics on the timescales up to tens Hz. UNIT will be operating by the middle of 2008.

astro-ph↗

arXiv2007

Detection of an oscillatory phenomenon in optical transient counterpart of GRB090522C from observations on Peak Terskol

22 Sep 2005 Swift-BAT triggered and located GRB050922C. The light curve shows the intense broad peak with $T_{90}$ of $(5 \pm 1)$ s. The Nordic Optical Telescope has obtained spectra of the afterglow with several absorption features corresponding to a redshift of $z = 2.17 \pm 0.03$. Observation of optical transient of GRB050922C was carried out in the R-band with the 60-cm telescope equipped with a CCD on Peak Terskol (North Caucasus). The OT magnitude was fading from R $\approx 16$ to $\approx 17.5$. Detection of an oscillatory phenomenon in the R post-burst light curve is described in this work. Analysis of the R data reveals coherent harmonic with a period of $0.0050 \pm 0.0003$ days (7.2 min) during observing run of about 0.05 days ($\sim 70$ min). Amplitude of oscillations is about 0.05 magnitude. The simplest model suggests that GRB050922C may result from tidal disruption of a white dwarf star by a black hole of about one thousand solar mass. The periodicity in the light curve can be identified with relativistic precession of an accretion disc.

astro-ph↗

arXiv2001

Millisecond Electron-Phonon Relaxation in Ultrathin Disordered Metal Films at Millikelvin Temperatures

We have measured directly the thermal conductance between electrons and phonons in ultra-thin Hf and Ti films at millikelvin temperatures. The experimental data indicate that electron-phonon coupling in these films is significantly suppressed by disorder. The electron cooling time $τ_ε$ follows the $T^{-4}$-dependence with a record-long value $τ_ε=25ms$ at $T=0.04K$. The hot-electron detectors of far-infrared radiation, fabricated from such films, are expected to have a very high sensitivity. The noise equivalent power of a detector with the area $1\mum^2$ would be $(2-3)10^{-20}W/Hz^{1/2}$, which is two orders of magnitude smaller than that of the state-of-the-art bolometers.

cond-mat.dis-nn↗